There's a decent chance that I'll be able to make it

I just want to give a bit of historical context about this trick, which maybe will make you feel a bit better:

The first time I saw this trick mentioned in the community was in the challenges board, I believe by ccexplore. Back then the culture was not to reveal details about obscure mechanics and have other people figure it out on their own. So when it was found the trick was just referenced as the Wild 15 trick/glitch (without description) and used in the maximum saved solution if I recall correctly. Challenges had been going on for a while then. It took a while for someone to find it and apply it in the first place, and even after the trick was referenced, most of the community wasn't aware what the Wild 15 trick was, even though we knew it was used in a specific level to save a certain amount of lemmings (and in principle could have been able to figure out). So while the trick might make sense in hindsight (blockers turn everything), it's not straight-forward to come up with it (as evidenced by its history described above). There's also no clear visual indication that a digger has been turned.

Nowadays it seems to be part of the standard repertoire of expert players, but I wonder what fraction of these players actually discovered it on their own. With challenge solutions floating around, and people playtesting each others' levels and sending solution and backroute replays, I think it's much more likely for someone to come across this trick by seeing it done by someone else than by discovering it on your own. The fact that you haven't found it in 15 years is less a matter of stupidity than not having come across it in someone else's replays you've watched. I would guess that most people would be in the same boat as you if they hadn't seen the trick in some replay or solution description previously.

Wow, great job finding all of these 100% solutions on your own, that's impressive. It took the community a long time to get there
(If you want to dig into the lore, until 2006, 2009-2012, the last two levels).

Nice find with the fast-forward glitch, too! So does it just skip every other frame, and the cycle length
of the tennis ball is even with one frame killing the lemmings? I knew fast forward was a bit wonky
with respect to physics, but never knew there were reliable patterns to exploit.
The pausing technique ccexplore mentioned is documented here.

Btw, do we happen to have all the tracks from Lemmings 2: The Tribes somewhere?

Attached is an archive with the Amiga, SNES and Genesis music (Circus is super catchy on the Genesis, we used to listed to that one on loop with Simon). Amiga I have in Octamed and mod format, should sound the same though. There are also midis for some reason, don't know where they come from. I also have DOS recordings with I think the Roland sound chip, but they are in ogg format and too fat to upload. Alas, I don't have my favourite version which is Adlib DOS (except for Cavelem).

There was a message today (2019-05-19) in IRC with an issue that might be related to the upgrade:

[02:09:38]   <JamesMcKigney> Greetings. I am not able to log on to the Lemmings forms and I can't get any emails from Lemmings Forums.
[02:10:22]   <JamesMcKigney> I have been trying to do so since yesterday.

What about this for the splat ruler? Though I think it's reasonable to also enable trigger area with it, and then the shades (or a wrench or something) might do.

For saving/loading states, not sure what the flag is meant to symbolize... if we have something that non-descript anyway, you might as well go for something more abstract like this: https://cdn.dribbble.com/users/71134/screenshots/1020838/save-icon.png
Still somewhat indicates storing something, but without explicitly invoking the disk imagery.

Less than 24 hours left to submit!

Good job! Pretty much all intended (for my levels anyway), except for some minor variations.
Thanks also for the difficulty estimates! I found African Wigwam quite tricky when I first played it.

I slightly modified Deadly Cells and Spin Me Around, see attached. Original proofs should still work.
Rock on the Block is also a tiny bit different, but definitely in the spirit of the original solution, no need to change anything there.